Tunable or switchable adhesion has a variety of applications including, e.g., the transfer printing of semiconductor elements, automated material handling, climbing robots, and medical tapes. There are a number of approaches relating to tunable adhesion, many of which include complex or difficult to manufacture geometries and/or the need for an external stimuli. Such approaches include variation of peeling speed, switching between modes of loading, structures with angled or asymmetric geometries, magnetic deformation, thermal actuation, humidity-driven changes, laser-based delamination, altering the real contact area through deformation, or a combination of these techniques.
For example, some schemes to achieve tunable adhesion have relied on fabricating posts or fibers with complex geometries, such as posts with mushroom-like caps. Fibrillar adhesives, which are often inspired by natural systems like the Gecko lizard or insects, use asymmetric post geometries to obtain adhesion control via peeling direction, similar to concepts in gecko adhesion. As one example, posts with one flattened side and one rounded side have been fabricated. When a shear load is applied to push the flat side into contact with the surface, a large contact area and high pull-off force are achieved. The opposite is true when shear load is used to push the rounded side into contact with the surface.
The mushroom-shaped geometry alters the stress distribution at the interface and shifts the peak stress from the edge of the post to the center of contact. This results in an increase in the force required to separate the post from the surface (i.e., the pull-off force) as it is more difficult to initiate a crack at the center than at the edge. However, tuning the adhesion of surfaces with mushroom-shaped posts can be difficult as the application of shear may not alter the stress distribution at the interface sufficiently to allow for a crack to initiate at the edge.
Thus, it would be useful to provide a tunable adhesive with a simple geometry that can be readily manufactured. There is also a need for a tunable adhesive that does not require an external stimuli (e.g., magnetic, thermal, pneumatic) beyond the applied loading. Further, it would be desirable to provide a tunable adhesive that offers strong adhesion under normal loading and weak adhesion when shear is applied.